Types of Circuit Breakers

Types of Circuit Breakers Points : Types of Circuit Breakers, Miniature Circuit Breakers, Earth leakage Circuit Breakers Types of Circuit Breakers
(a) Miniature Circuit Breakers
(b) Earth leakage Circuit Breakers (a) Miniature Circuit Breakers Every electrical circuit and final circuit shall be protected against over current by suitable over current protective devices such as miniature Circuit Breakers.

The M.C.B’s is mounted in a D.F.H. The M,C.B’s work automatically when current passing through them exceeds the value for which they are set. M.C.B’s is designed in this way that it has a time delay type characteristics. The operating time of miniature circuit breaker is controlled by the magnitude of the over current which passing through it. Another characteristic of M.C.B’s is that it cannot operate at transient overload such as motor starting currents and switching surges. The breaker only operates if the overload is present for a long time which is dangerous for C.B’s.

According to lEE regulations state that the M.C.Ws must operate at an O/L not exceeding 1.5 times the designed load current of the circuit with in 4 hours.

The automatic operation and time-delay characteristics can be obtained in two ways, magnetic tripping or thermal tripping.

(i) Magnetic Tripping: Electro magnet is a coil, when electric current passing through it become a magnet. At normal running current, the electromagnet flux is not strong or not sufficient to attract the arm, but when overloading or short circuit occur, the magnetic flux is increased and the magnetic force become strong as a result the magnet pull is increased. The arm is attracted to the magnet and trip circuit is complete and C.B disconnects the supply. (ii) Thermal Tripping: Automatic operation, with a time delay characteristic is achieved in this’ case by means of a thermally bi-metallic element.

In normal running condition C.B work properly. When the overload or short circuit occurs, the value of current increased, as a result the bi-metallic element is heated to a given temperature, and C.B is tripped. Thermal-type units are effected by ambient temperature. Therefore, when this type of C.B is once tripping it is necessary to be left the C.B for a short time to enable the bi-metal. (b) Earth leakage Circuit Breakers Rapid and quick disconnection of circuit for protection against shock is achieved by the use of earth leakage C.B. The method of operation is as follows.

This type of C.B’ has a primary and secondary coil which is wound on a magnetic core. The current in both the phase and neutral Conductors are passed through the C.B. In normal condition the value of current in primary and secondary coils are equal, therefore the fluxes are cancelled to each other and there is no induced current in the trip coil of CB. As a result the CB works in normal condition. If an earth fault occurs in the circuit, the currents in primary and secondary coils arc not equal or no longer balance. As a result, a current is induced in trip coil. The trip coil disconnects the circuit through supply mains by opening the main contacts.

Cut-away section of a typical miniature circuit breaker. Under overload conditions a bi-metal blade operates the trip lever (A) which releases the contacts (B). Under short circuit conditions, the solenoid (C) is energized which opens the contacts (8) rapidly by means of plunger. (D). The arc which occurs between the parting contacts is moved magnetically along the arc runners (E) and is split up and extinguished by the arc splitter pack (F).